This invention relates to radiation receiving systems and, more particularly, to a monopulse receiving system incorporating a microwave preselector to provide image rejection and power limiting.
As is known in the art, the subfeedthrough or subclutter visibility of a radar receiver operating in a pulse Doppler mode is limited by the pulse Doppler spectral line existing at the image, or second harmonic frequency, in the envelope of the spectrum of I.F. frequencies existing in such radar receiver. Thus, for example, if the envelope of the spectrum of a pulse Doppler illuminator is down by 65 decibels at 60 MHz from the center frequency of such spectrum, and if a radar receiver employing a 30 MHz I.F. frequency is required to provide 90 decibels of subfeedthrough visibility, at least 25 decibels image rejection must be provided. Further, when monopulse tracking is a requirement, any image rejection means which is employed in the sum and difference channels must be arranged so that no changes in the relative phases and amplitudes of the signals in the different channels occur. That is, "amplitude and phase track" is essential to avoid unacceptable degradation of tracking accuracy.
One technique used to provide image rejection is to employ known "image rejection" mixers to provide down conversion from the R.F. to I.F. frequencies. Conventional image rejection mixers, however, cannot provide the degree of image rejection required. To provide adequate image rejection, known so-called "short circuit" image rejection mixers may be used provided the local oscillator may supply the requisite amount of radio frequency power. Whether an image rejection mixer or short circuit image rejection mixer is used, a microwave bandpass filter and a diode limiter are required in circuit with the mixer to prevent burnout when jamming signals from an enemy radar or interference signals from a friendly source are experienced. Unfortunately, however, such required bandpass filter and limiter will reduce the overall noise figure of the radar receiver.
Another common technique used to provide image rejection in a monopulse receiver is to employ a three channel, double I.F. frequency conversion receiver with a narrow band filter at the first I.F. frequency to reject images at the first I.F. frequency. The narrow band I.F. filters used for such a purpose must, however, be quite large in order to provide the degree of filtering required. The double I.F. conversion technique is, therefore, impractical in applications wherein the size of the receiving system is critical, as when such receiver is to be used in a missile. In addition, a microwave bandpass filter and limiter are still required for protection against undesired electromagnetic radiations such as jamming or interfering signals.
Still another technique used to provide image rejection in a monopulse receiver is to employ a two channel, single frequency conversion receiver with an electrically tunable preselector utilizing single crystal spheres or discs of yttrium-iron-garnet in a microwave resonator (such a resonator being referred to as a YIG filter). In such a system the two monopulse difference signals are time multiplexed through a single receiver channel and the use of the YIG filter in a preselector obviates the need for a microwave bandpass filter and limiter. However, a single pole double throw diode switch, which is required to provide the switching between the two difference channels, adds to the system noise figure and, in addition, the time multiplexed system provides inferior ECM performance in comparison with a three channel monopulse system.